This application relates to an improvement for a hydrostatic steering system which includes a fluid source, a steering motor, and a hydrostatic steering controller which is operated by a steering wheel and controls fluid flow from the source to the steering motor. It relates particularly to an improvement for a system such as a load sense system which increases flow and pressure delivered from the source to the controller during a steering maneuver. More specifically, it relates to a special coupling between the hydrostatic steering controller's metering unit and the steering wheel, which coupling minimizes the possibility of a high reaction torque being felt by a vehicle operator if flow and pressure delivered from the source to the controller are insufficient to effect a steering maneuver when the operator effects steering.
Examples of load sense hydrostatic steering systems are shown in U.S. Pat. Nos. 3,931,711 and 4,079,805 and pending U.S. application Ser. No. 243,497. Each of the systems includes a hydrostatic steering controller which is operated by the vehicle operator. The steering controller includes a metering unit which meters flow to the steering motor and a control valve which ports fluid from the fluid source to a metering unit, from the metering unit to the steering motor, and from the steering motor to a reservoir.
When there is no steering taking place, a minimum standby flow and pressure is communicated from the fluid source to the controller, and the control valve is in a neutral position. When the operator turns the steering wheel, the control valve moves away from the neutral position to an operating position and flow from the fluid source is increased, and directed to the metering unit. The flow is metered and then directed to the steering motor.
As the operator turns the steering wheel, the effort with which he turns the wheel is transmitted from an input member to the metering unit. Operation of the metering unit is resisted by the steering load which varies depending on various conditions including, e.g., whether the vehicle wheels are on ice, mud, etc. Thus, during steering, a reaction force is transmitted from the metering unit to the input member in accordance with the steering load resistance. The flow and pressure directed to the controller assist the metering unit in overcoming the steering load resistance, and directing metered amounts of fluid to the steering motor.
In the load sense system of U.S. Patent application Ser. No. 243,497, a pilot conduit directs a pilot flow of fluid through the controller to a reservoir when the control valve is in the neutral position. When steering occurs, the pilot flow is restricted, and a load sense signal is directed to a means which increases the flow and pressure delivered to the controller from the fluid source. The system of application Ser. No. 243,497 reacts quickly to turning of the steering wheel to provide the pressure signal for increasing flow and pressure delivered from the fluid source to the controller.
Nevertheless, despite the speed with which the system of application Ser. No. 243,497 reacts, there is still a time interval between the creation of the pressure signal and the point at which the increased flow and pressure are actually delivered to the controller. This time interval results from factors such as (1) the vehicle operator making so high a demand that the system simply cannot react fast enough to deliver the amount of flow and pressure demanded, and (2) the inertia of the mechanism which responds to the pressure signal to increase the flow directed from the source to the controller.
In the event that the operator makes a very high demand (i.e., a very rapid and forceful turning of the steering wheel), a sudden and unexpected reaction torque can be felt by the operator during the time interval it takes for the flow and pressure being directed to the controller to increase to the demanded levels. Specifically, if flow and pressure delivered to the controller were below demand levels as the valve reaches an operating position, the steering load resistance acting on the metering unit would cause a sudden increase in the reaction torque transmitted to the input member. That reaction torque, if transmitted directly to the steering wheel would be felt by the operator.